The aluminum electrodeposition potential is lower than the hydrogen evolution potential, and it thus is impossible to electrodeposit aluminum from its aqueous solution. For this reason, as aluminum electroplating solutions, those using a nonaqueous solvent have been often studied. As nonaqueous solvents, tetrahydrofuran, toluene, and the like are known. However, such solvents are problematic in that they are highly inflammable, and, therefore, most of them are not in actual use. Under such circumstances, as a relatively safe aluminum electroplating solution, Patent Document 1 reports a low-temperature molten salt electroplating solution prepared by mixing and melting dimethyl sulfone and an aluminum halide (aluminum chloride, etc.). Nevertheless, because the plating solution uses dimethyl sulfone as a nonaqueous solvent, the bath make-up cost is high. Therefore, in order to reduce the plating cost, it is necessary to extend the life of the plating solution. However, because the plating solution uses an aluminum halide of high hygroscopicity as a solute, it has the property of gradually absorbing moisture from the air, resulting in degradation. When a plating process is performed using a plating solution degraded due to the absorption of moisture, this is likely to cause the formation of a black film called burnt deposit (hereinafter simply referred to as “burnt deposit”).
In an attempt to solve the above problems of the aluminum electroplating solution described in Patent Document 1, the present inventors have reported, in Patent Document 2, a method in which a plating solution is configured to contain dimethylamine borane so as to effectively remove moisture from the plating solution. However, a subsequent study on this method has revealed that when an increased amount of moisture is incorporated into the plating solution, this may cause a rapid reaction between dimethylamine borane and water, resulting in the ignition of the plating solution.